Mine shafts sometimes experience cave-ins, collapses, or falling rock due to the layered and stratified makeup of the earth. A mine shaft itself may cause fractures and weaknesses in a strata in its ceiling, or it may just expose an inherently weak and unstable layer. To assist in preserving the integrity of the ceiling, it is common to support the ceiling with bolts anchored up rock layers above the ceiling. Plates between the bolt heads on the exposed ends of the bolts and the ceilings are used to transfer force from the anchored bolts to the exposed layer of the ceiling. In some applications, the exposed end of the anchored bolt is threaded. Onto these bolts, a nut is threaded, and the nut is used to place a preload on the bolt to set an initial lifting force to the plates.
Holes, which are slightly oversized to the bolts, are drilled into the ceiling. Sometimes the holes must be several feet deep to be sure of anchoring the bolts in a stable layer of rock. Once the holes are drilled, the bolts are inserted into the holes and anchored. There are three methods for anchoring the bolts in the holes, mechanical, adhesive, and mechanically assisted adhesive. This patent relates mostly to the mechanical method or the mechanical aspect of the mechanically assisted adhesive method of anchoring bolts, so the adhesive method will be discussed only briefly before discussing the relevant mechanical art.
Once the hole is drilled, a multi-component adhesive is placed in the blind end of the hole. The components of the adhesive are kept in separate frangible packages to keep them from mixing, for once they do, a reaction occurs, and the adhesive begins to set up. The components of the adhesive are usually a hardener and a catalyst. When the frangible packages have been placed in the hole, a bolt is inserted and turned rapidly to rupture the packages and thoroughly mix the adhesive components. The adhesive is typically of a fast setting variety and may begin to set after three to five seconds of mixing. For many mechanical anchoring methods, the mechanical anchoring elements on the bolt assist in mixing the adhesive, and the increased resistance to mixing of the setting adhesive activates the mechanical anchoring system.